This invention relates in general to float zone refining of semiconductor material and in particular to improvements in the initiation of the inductive heating process used for such refining.
In a typical apparatus for the float zone refining of semiconductor material such as silicon, induction heating using a radio frequency (RF) energy source is common. To heat silicon effectively by induction requires much higher frequency than for metal and thus inductive heating means are used which deliver energy at a frequency of from 1 to 10 megahertz. Typically, the RF energy is generated by the resonant circuit of a high power oscillator circuit and delivered to an induction heating coil which has a small enough coil diameter to produce a high induction field. High inductive fields are required because the resistivity of ultra-pure monocrystalline or polycrystalline silicon at room temperature is high and it is thus difficult to start the inductive heating. When inductive heating is initiated, the resistivity of the silicon decreases as the temperature increases. This has the effect of making the inductive heating progressively more effective since as more of the RF energy applied to the inductive heating coil is "coupled" into the semiconductor material being heated, the reduction in resistivity further enhances the coupling. The accelerated effect of this process continues until portions of the inductively heated silicon reach the melting point at which time there is a step function reduction in the resistivity of the silicon by a factor of more than 20. The corresponding sudden increase in coupling causes a very rapid temperature rise which in turn causes severe thermal shocks. The float zone refining process is typically initiated by positioning the pointed end of a relatively large diameter polycrystalline silicon feed rod in the center of an inductive heating coil and adjacent to a seed crystal. When inductive heating is initiated, the temperature of the pointed end of the feed rod quickly rises to the melting point while the remaining larger diameter portion of the feed rod stays relatively cool. The thermal shock associated with the large temperature differential between the pointed end and the bulk of the rod can create severe problems such as shattering of the polycrystalline feed rod.